Four tents were burnt in the attacks, injuring several protesters. One police officer and six soldiers, deployed to the scene to arrest the attackers, were wounded by rubber bullets, according to the interior ministry. The health ministry said a total of 15 people were injured.

Protesters opposing an Islamist-backed constitution have been camped outside the residence of President Mohamed Mursi for weeks. Around 20 chanted "Down, down Mursi!" and "We will not leave!"

In December, Mursi signed into law the first constitution since the overthrow of veteran autocrat Hosni Mubarak in a 2011 revolution.

The text, which Mursi fast-tracked through parliament and a national referendum, was fiercely opposed by many of the liberals, secularists and leftists who had helped to topple Mubarak, and has prompted often violent protests on the streets of Cairo.

A Reuters reporter saw several ambulances driving away from the scene late on Saturday evening, and plumes of black smoke billowed in the air.

"The police and military police have been negotiating with us since yesterday to leave the square," said protester Karim el-Shaer. "Today suddenly and only half an hour after the negotiations failed, we found thugs attacking us and burning our tents under the eyes of the security forces."

Access to rubber bullets and guns is easy in post-revolution Egypt given the lax security and a rise in arms smuggling. The interior ministry said it had launched an investigation.